That seems it would constitute and update every time you touch the document. In other words, Word would have to capture the fact you added a page or deleted a page.
The typical way would be to select Ctrl + A and then F9...
but is this is not what you want then you could do it through a macro.
Code:
Sub Test()
ActiveDocument.Fields.Update
End Sub
I'm sure Charles can add some additional details to cover, but maybe this helps in some way.